The present disclosure relates to a fiber Bragg grating (FBG) pressure sensor assembly and a method of using it.
Fiber-optic sensors are increasingly being used as devices for sensing some quantity, typically temperature or mechanical strain, but sometimes also displacements, vibrations, pressure, acceleration, rotations, or concentrations of chemical species. The general principle of such devices is that light from a laser is sent through an optical fiber and there experiences subtle changes of its parameters either in the fiber or in one or several fiber Bragg gratings and then reaches a detector arrangement which measures these changes.
The growing interest in fiber optic sensors is due to a number of inherent advantages:                Inherently safer operation (no electrical sparks)        Immunity from EMI (electromagnetic interference)        Chemical passivity (not subject to corrosion)        Wide operating temperature range (wider than most electronic devices)        Electrically insulating (can be used in high voltage environment)        
One of the most commonly used and broadly deployed optical sensors is the fiber Bragg grating (FBG), which reflects a wavelength of light that shifts in response to variations in temperature and/or strain. FBGs are constructed by using holographic interference or a phase mask to expose a short length of photosensitive fiber to a periodic distribution of light intensity. The refractive index of the fiber is permanently altered according to the intensity of light it is exposed to. The resulting periodic variation in the refractive index is called a fiber Bragg grating.
When a broad-spectrum light beam is sent to an FBG, reflections from each segment of alternating refractive index interfere constructively only for a specific wavelength of light, called the Bragg wavelength, described in equation (1). This effectively causes the FBG to reflect a specific frequency of light while transmitting all others.λb=2nΛ  (1)
In equation (1), λb is the Bragg wavelength, n is the effective refractive index of the fiber core, and Λ is the spacing between the gratings, known as the grating period. The reflected wavelength λb is affected by two principle mechanisms 1) changes in strain on the fiber that alters the grating period as the spacing between the gratings moves, and 2) changes in temperature that alter the refractive index.
FBG strain sensors are thus somewhat complex because both temperature and strain influence the sensor's reflected wavelength. For proper strain measurements, you must compensate for the temperature effects on the FBG. You can achieve this by installing an FBG temperature sensor in close thermal contact with the FBG strain sensor. A subtraction of the FBG temperature sensor wavelength shift from the FBG strain sensor wavelength shift can then yield a temperature compensated strain value. As will be shown however, this approach can have resolution and accuracy issues.
The purpose of this disclosure is to describe a configuration and method that permits the creation of a temperature-compensated pressure gauge for downhole use that is based on Fiber Bragg Gratings and can be interrogated with higher resolution and higher accuracy than by previous methods and with the additional benefit of being less sensitive to error-inducing drift factors such as FBG thermal degradation and hydrogen attack.
This technique can be useful for any strain-based transducers that also need temperature compensation.
To simplify the description, however, we will use pressure as the measurement of interest for the strain-based transducer in the rest of this document.